This invention relates generally to solid disk surface unit assemblies for mounting in cooktops having a glass or ceramic support surface and in particular to an improved thermal protection arrangement for such assemblies.
Cooktops and ranges featuring solid disk surface units of cast metal material mounted in a sheet metal support surface have been popular for many years particularly in Canada and Europe. In recent years cooktops featuring solid disk surface units mounted in a tempered glass support surface have been introduced to the market place in this country. An arrangement for mounting conventional solid disk surface units in a glass or ceramic support surface is described in U.S. Pat. No. 4,491,722 to Fischer et al.
As described in U.S. Pat. No. 4,122,330 to Fischer, conventional solid disk units may be provided with switching devices which are directly responsive to the temperature of the cast iron heating surface of the surface unit and operative to interrupt energization of one or more of the heating elements in the unit to prevent the unit from becoming hot enough to cause warpage of the unit or of pans supported on the unit. Set point temperatures on the order of 900.degree. F. are typical for such switches. Since a loaded pan placed on a surface unit acts as a heat sink at the contact points, temperatures high enough to damage the cast metal surface units are likely to occur only when the unit is operating at its maximum power setting with either no pan or an empty pan on the unit. Use of thermal limit switches responsive to the temperature of the heating surface in the manner described works satisfactorily in such extreme conditions to prevent damage to the disk itself. In appliances in which the units are supported from a sheet metal support surface, protection against such extreme operating conditions is sufficient. Furthermore, with good pans, that is, pans that provide good uniform surface contact across the entire heating area of the solid disk surface unit, such an arrangement is adequate even with a glass support surface. However, pans, particularly relatively inexpensive pans, tend to become warped with extensive use and warped pans permit air gaps to exist between the pan and the cast iron heating surface. When such gaps exist the heat is not conducted away from the exposed portions of the surface unit and localized hot spots result. The temperature of the glass cooktop surface in the vicinity of such a hot spot may rise to a temperature near or even exceeding the annealing temperature for the glass. When such conditions exist, internal stresses are created in the glass in the vicinity of the hot spot. Repeated cycling of such conditions eventually causes glass fatigue resulting in a shattering of the glass. Due to the localized nature of such hot spots, the temperature of the regions of the surface unit in good contact with the pan may remain well within acceptable temperature limits. Thus, hot spot conditions sufficient to cause such damage may go undetected by the protective arrangement above described unless the hot spot happens to occur in the immediate vicinity of the internal switch sensor.
Hence, there is a need for a protective arrangement for cooktops featuring solid disk surface units mounted in glass support surface, which can respond to hot spots occurring anywhere around the periphery of the surface unit and interrupt the power to the surface unit before the temperature of the glass in the vicinity of the hot spot rises to a level sufficient to damage the glass.
It is therefore an object of the present invention to provide a thermal protection arrangement for a solid disk surface unit mounted in a glass support surface which responds to the occurrence of hot spots anywhere around the periphery of the disk and reduces the power to the surface unit before the temperature of the glass in the vicinity of the disk rises to a level sufficient to cause damage to the glass, without interfering with normal operation of the surface unit.
It is a further object of the present invention to provide a thermal protection arrangement of the aforementioned type which can be incorporated in present solid disk surface unit asemblies at low cost with only minor changes to the mounting structure and circuitry.